Help - cutting 2.5V/200mA xmas light string to custom length on 120VAC?

[Ryan Andersen]

I'm relatively clueless about component-level stuff, but am faced with a
once-off hobby project that will require very basic "circuitry" to realize
and am hoping you folks can assist.

Am I correct in thinking that a single resistor would be all that's needed
to reduce 120VAC to 10VAC for the purpose of cutting a string of series-
wired 2.5V/200mA incandescent mini Christmas lights down to only 4 bulbs?
Those strings have no such "circuitry" to speak of pre-fabbed, as they are
usually manifactured in 48-50 bulb lengths (120V / 48-50 = 2.5V-2.4V each).

Problem is, I don't know how to calculate the correct resistor value. Nor
am I certain that one resistor would be a safe approach (excess heat if just
one?). Nor am I even certain that resistor(s) are all I'd need, as opposed
to multiple component types.

Here's what I'm doing. Cue antique doorbell fixture:

http://img36.imageshack.us/img36/6463/img1054xf.jpg

Opened, I found the anticipated DC doorbell circuit's wiring, but also, a
working 120VAC pair, evidently for a backlight that no longer exists:

http://img29.imageshack.us/img29/1504/img1060xy.jpg

I'm currently in the process of cleaning it up and replacing the wiring --
especially the old lamp chord (!) used for the AC. (Fortunately, it ties
into an indoor light switch housing directly opposite the doorbell fixture,
and will be a snap to replace in its entirety.)

Anyway, experimenting with the fixture's face and possible backlighting
methods, I found that standard "night light" bulbs didn't fit within the
enclosing very well (and ran too hot), and that LEDs looked rather ugly
(their light is simply too directional). What wound up working ideally
were 2.5V mini Christmas bulbs, if arranged thusly behind the address
mask's translucent backplate:

http://img183.imageshack.us/img183/2113/23767119.png

The benefits, besides appearance, would be lowest possible heat for any
incandescent-type bulb, and extremely cheap replacement bulbs -- already
have hundreds of white ones on hand now.

So I'm thinking of doing something like shown by this mock-up (ignore the
resistor value; it's just clipart taken randomly from the internet):

http://img36.imageshack.us/img36/2139/50829898.png

Additionally picture securely soldered connections and some nice, clean
shrink tubing, and bingo.

Anyone? P.S. Would love to know how to calculate the resistor value in
leu of simply being told which to use for 10VAC. For all I know, I may
wind up going with slightly more or fewer bulbs than 4.

 

[ehsjr]

I'm relatively clueless about component-level stuff, but am faced with a
once-off hobby project that will require very basic "circuitry" to realize
and am hoping you folks can assist.

Am I correct in thinking that a single resistor would be all that's needed
to reduce 120VAC to 10VAC for the purpose of cutting a string of series-
wired 2.5V/200mA incandescent mini Christmas lights down to only 4 bulbs?
Those strings have no such "circuitry" to speak of pre-fabbed, as they are
usually manifactured in 48-50 bulb lengths (120V / 48-50 = 2.5V-2.4V each).

Problem is, I don't know how to calculate the correct resistor value. Nor
am I certain that one resistor would be a safe approach (excess heat if just
one?). Nor am I even certain that resistor(s) are all I'd need, as opposed
to multiple component types.

Here's what I'm doing. Cue antique doorbell fixture:

http://img36.imageshack.us/img36/6463/img1054xf.jpg

Opened, I found the anticipated DC doorbell circuit's wiring, but also, a
working 120VAC pair, evidently for a backlight that no longer exists:

http://img29.imageshack.us/img29/1504/img1060xy.jpg

I'm currently in the process of cleaning it up and replacing the wiring --
especially the old lamp chord (!) used for the AC. (Fortunately, it ties
into an indoor light switch housing directly opposite the doorbell fixture,
and will be a snap to replace in its entirety.)

Anyway, experimenting with the fixture's face and possible backlighting
methods, I found that standard "night light" bulbs didn't fit within the
enclosing very well (and ran too hot), and that LEDs looked rather ugly
(their light is simply too directional). What wound up working ideally
were 2.5V mini Christmas bulbs, if arranged thusly behind the address
mask's translucent backplate:

http://img183.imageshack.us/img183/2113/23767119.png

The benefits, besides appearance, would be lowest possible heat for any
incandescent-type bulb, and extremely cheap replacement bulbs -- already
have hundreds of white ones on hand now.

So I'm thinking of doing something like shown by this mock-up (ignore the
resistor value; it's just clipart taken randomly from the internet):

http://img36.imageshack.us/img36/2139/50829898.png

Additionally picture securely soldered connections and some nice, clean
shrink tubing, and bingo.

Anyone? P.S. Would love to know how to calculate the resistor value in
leu of simply being told which to use for 10VAC. For all I know, I may
wind up going with slightly more or fewer bulbs than 4.

You have a different problem to address first:
The lampcord connection of 120 VAC to your doorbell is unsafe. It also
violates the electrical code. You can't run lampcord as permanant wiring
per the code, and having 120 volts behind your doorbell button is not
safe.

Now, lets get to your question. If you drop 110 volts in a resistor to
feed 10 volts to your lamps at 200 mA, the resistor will produce about
22 watts of heat. You don't want to do that - it'll be way too hot.
You can solve all the problems and have a safe installation if you
use a wall wart supply. Allelectronics CAT# DCTX-532 is a regulated
5.5 volt DC supply for $3.50. Wire the + side to the non-banded end
of a 1N4001 diode. Wire the banded end to your lamps, which you wire
as two strings of series bulbs in parallel, like this:

-------- 1N4001
| DCTX +|---->|----+---[Lamp]---[Lamp]---+
| 532 | | |
| Wall | +---[Lamp]---[Lamp]---+
| Wart | |
|Supply -|--------------------------------+
--------

You do not need a resistor for the above arrangement. The diode
will reduce the voltage by about .7 volts, and running your
bulbs a little bit low (about 2.4 volts each) is ideal.
Allelectronics also sells the diode at $1.00 for 15.
http://www.allelectronics.com/

You asked about computing resistance to drop voltage.
To compute the resistance needed, you use the formula R = E/I where E
is the voltage you want the resistor to drop, I is the current, and R
is the resistance, in ohms. For example, to drop 1 volt at 200 mA you
need a 5 ohm resistor: R = 1/.2

Ed

 

[default]

I'm relatively clueless about component-level stuff, but am faced with a
once-off hobby project that will require very basic "circuitry" to realize
and am hoping you folks can assist.

Am I correct in thinking that a single resistor would be all that's needed
to reduce 120VAC to 10VAC for the purpose of cutting a string of series-
wired 2.5V/200mA incandescent mini Christmas lights down to only 4 bulbs?
Those strings have no such "circuitry" to speak of pre-fabbed, as they are
usually manifactured in 48-50 bulb lengths (120V / 48-50 = 2.5V-2.4V each).

Problem is, I don't know how to calculate the correct resistor value. Nor
am I certain that one resistor would be a safe approach (excess heat if just
one?). Nor am I even certain that resistor(s) are all I'd need, as opposed
to multiple component types.

Here's what I'm doing. Cue antique doorbell fixture:

http://img36.imageshack.us/img36/6463/img1054xf.jpg

Opened, I found the anticipated DC doorbell circuit's wiring, but also, a
working 120VAC pair, evidently for a backlight that no longer exists:

http://img29.imageshack.us/img29/1504/img1060xy.jpg

I'm currently in the process of cleaning it up and replacing the wiring --
especially the old lamp chord (!) used for the AC. (Fortunately, it ties
into an indoor light switch housing directly opposite the doorbell fixture,
and will be a snap to replace in its entirety.)

Anyway, experimenting with the fixture's face and possible backlighting
methods, I found that standard "night light" bulbs didn't fit within the
enclosing very well (and ran too hot), and that LEDs looked rather ugly
(their light is simply too directional). What wound up working ideally
were 2.5V mini Christmas bulbs, if arranged thusly behind the address
mask's translucent backplate:

http://img183.imageshack.us/img183/2113/23767119.png

The benefits, besides appearance, would be lowest possible heat for any
incandescent-type bulb, and extremely cheap replacement bulbs -- already
have hundreds of white ones on hand now.

So I'm thinking of doing something like shown by this mock-up (ignore the
resistor value; it's just clipart taken randomly from the internet):

http://img36.imageshack.us/img36/2139/50829898.png

Additionally picture securely soldered connections and some nice, clean
shrink tubing, and bingo.

Anyone? P.S. Would love to know how to calculate the resistor value in
leu of simply being told which to use for 10VAC. For all I know, I may
wind up going with slightly more or fewer bulbs than 4.

If I'm reading this correctly, you do have a source of low voltage DC
inside the fixture? Use it, if you can and run the lights from the
low voltage source.

The thing looks it was designed for a back light . . . It is (and was)
common for that type of combined doorbell/house number fixture to use
low voltage filament bulbs running at reduced voltage and current in
series with the bell push. That is the lights are on until the
pushbutton is depressed then they go off while the button is held down
and the inside chime strikes.

The chime strike solenoid coil is in series with the lamp and some
small current flows through the coil at all times (but not enough to
pull in the plunger and ring the chime). Pushing the button shorts
the light and the current through the solenoid increases dramatically
and the chime strikes.

Trying to drop 120 VAC to 10 volts 200 ma is wasteful power wise.
Twenty watts doesn't sound like much but over time it represents a
fair amount of heat (just like a 20 watt soldering iron).

It costs money and it isn't safe. The door bell circuits already have
a step down transformer (usually AC not DC operated) for the chimes or
bell coils. ehsjr already gave you the formula to use and riot act
regarding the practice you are contemplating . . .

Ideally, size the lights to run on the existing low voltage supply
with no resistor.

Now regarding Christmas tree filament light bulbs - not your best
choice. Most of them are designed for series string and have some
fuse in there to protect the wiring - you would also need to include a
fuse to limit the current to 1/2 amp or so.

Series string Christmas lamps are designed with a small shorting bar
arrangement in the bulb. If the bulb filament burns out it shorts the
light so the others in the string remain on. With only four lamps . .
.. if one burns out the other three will get very bright for a time
until they drop dead and short out - and eventually something else
will open to remove current from the short - hopefully without
starting a fire (that's where the fuse comes in).

Or just invest a few bucks and buy some miniature bayonet sockets and
wire in some 12 volt bulbs.
--